I finally sorted the iffy heater control valve cable! The better part of a day's fannying about but the controls now have a very light action. Why VWSA never took this approach in the first place I can't imagine...
The problem. SA has a special cable to connect to the Audi 80 control panel and it is far too short imho, so has a tortuous path down to the valve which ended up with it getting kinked:

I had a new rhd one, but it needed the outer shortening to achieve the correct end-to-eye dimension as the original. This meant cutting the end off the SA outer and hooking out the inner with a small drill:

Then the outer could be trimmed back (35mm in the end, but an inch more wouldn't matter as you will see later)

Then the locating tag could be refitted to the rhd cable and a new eye formed (not easy!)

The new cable was a fiddle to refit but the control panel will roll over just enough to give access. Take out the dash pod, radio and heater ducting for access. You'll need it!

The cable still had a poor approach angle to the valve , so I drilled a new hole at the "corner" of the tunnel where it would still be hidden by the heater ducting:

From underneath:

The cable now runs under the radio, past the vaccum actuator and in a large arc down to the grommet and in a straight line to the valve. It is hardly visible from the passenger seat, hence the comment that it could be an inch shorter without any worries.

All well worth the effort and eliminates another 22S part.
PS remove the top 450mm of the rubber star-section anti-rattle sheathing from the cable, the reduced bulk helps it settle back under the radio and achieve a more gentle line.
Finally I swapped the awful T3 "beeper" for some mk3 golf horns. Much more German-sounding!

1985 Oettinger 3.2 Caravelle RHD syncro twin slider. SA Microbus bumpers, duplex winch system, ARC 7X15 period alloys